1. Field of the Invention
The present invention generally relates to an image forming apparatus, such as a copier, printer, fax machine, or the like, that uses an electrophotography process. More specifically, the present invention relates to a laser scanning unit having a beam splitter which is capable of splitting a laser beam with high efficiency, and a tandem image forming apparatus having the laser scanning unit.
2. Description of the Related Art
In general, a laser scanning unit forms an electrostatic latent image, according to video data to be printed, by scanning a modulated laser beam onto the surface of a photosensitive drum.
FIG. 1 illustrates a conventional laser scanning unit for the purpose of explaining a laser scanning process. Referring to FIG. 1, in the laser scanning unit, laser beams 1 emitted from a light source 10 such as a laser diode 11 pass through a collimating lens 12 and a cylindrical lens 13, and are reflected and deflected by a polygon mirror 20 driven by a driving means 30. The deflected laser beams pass through an fθ lens 40, and are scanned horizontally (main scanning direction) onto the surface of a dielectric photosensitive drum 60 by a reflecting mirror 50 to form a latent image. The drum 60 rotates at a constant speed for the perpendicular (sub-scanning direction) scanning of a page.
In a color image forming apparatus, a plurality of laser beams generated from a light source are scanned onto a plurality of photosensitive drums corresponding to respective colors. Therefore, it is important that a laser scanning unit used in the image forming apparatus is capable of splitting the laser beams very efficiently.
FIGS. 2a and 2b illustrate the configuration of laser scanning units used in a tandem image forming apparatus, which is an example of a color image forming apparatus. Generally, a tandem color image forming apparatus has one of two types of laser scanning units. In the first type of unit, shown in FIG. 2A, a plurality of laser beams 1 pass through a plurality of polygon mirrors 20Y, 20M, 20C, 20K and their corresponding fθ lenses 40Y, 40M, 40C, 40K, and form latent images on photosensitive drums 60Y, 60M, 60C, 60K, respectively. Since each of the driving means 30Y, 30M, 30C, 30K and each of the lenses 40Y, 40M, 40C, 40K have their own properties and the laser scanning unit needs to be adjusted for each of them, the total number of components required is large and the control of the laser scanning unit is difficult. In the second type of laser scanning unit, illustrated in FIG. 2B, a plurality of laser beams 1 pass through a single polygon mirror 20a (which is driven by a driving means 30a) and a plurality of fθ lenses 40Y, 40M, 40C, 40K, and form latent images on the photosensitive drums 60Y, 60M, 60C, 60K, respectively. In this case, a deflecting means may be shared by deflecting laser beams to a single polygon mirror simultaneously. However, despite the use of the fθ lens group and the reflecting mirror group 50Y, 50C, 50K for separating the laser beams, the space between the beams is small, and the control of the laser beams is restricted due to the limited space. Further, the configuration of the image forming apparatus remains relatively complicated.
Accordingly, there is a need for a laser scanning unit which provides increased space between laser beams with a simple configuration.